In prior art hybrid propulsion systems, an internal combustion engine is used for driving a pump. The pump pressurizes a working fluid, specifically an incompressible fluid such as hydraulic fluid. The pressurized fluid is supplied through appropriate control circuitry to a hydraulic motor, such as a swash-plate motor. The swash-plate motor can be selectively coupled to wheels, tools, a cooling system, or other power means associated with an engine-driven machine, such as bulldozers, excavators, motor graders, and other types of heavy equipment, in order to drive the wheels, tools, cooling system or other power means of the equipment.
It is known that in hybrid propulsion systems, the fuel combustion engine may be called upon to deliver more power than the engine is designed to deliver or may even be shut down in order to conserve fuel. During this time of engine power shortage or passive engine operation the main transmission pump stops pressurizing the hydraulic fluid in the transmission or hybrid transmission. However, the components within the transmission must still receive a flow of pressurized hydraulic fluid in order to maintain operability. Current hybrid systems use a motor driven pump during engine down time for this purpose of delivering a pressurized hydraulic fluid flow to these components, in order to keep these components engaged so that the transmission is ready to respond. The pump may be powered by an electric motor or accumulators.
Prior art accumulator powered systems illustrate the importance of maintaining the accumulator of a hydraulic power system at a charge of energy which is sufficient to meet the needs of the equipment and in a manner which is cost-effective and environmentally friendly.
One of the power drains in an integrated hystat fan and hybrid system is the cooling system which typically comprises one or more air-to-air and/or liquid-to-air heat exchangers that chill coolant circulated through the engine and combustion air directed into the engine. In the cooling system, heat from the coolant or combustion air is passed to air from a fan that is speed controlled based on a temperature of the engine and based on a temperature of an associated hydraulic system. Although effective at cooling the engine, it has been found that the electro-hydraulic system driving the cooling fan may have excess capacity at times that is not utilized or even wasted. With increasing focus on the environment, particularly on machine fuel consumption, it has become increasingly important to improve upon the efficiency of electro-hydraulic charging systems in order to fully utilize all resources in the integrated hystat fan and hybrid system.
One attempt to improve electro-hydraulic system charging efficiency is described in related application Ser. No. 12/957,094 of inventors Bryan Nelson et al., filed Nov. 30, 2010 and assigned to Caterpillar, in which a hydraulic fan circuit is disclosed having a primary pump, a high- and a low-pressure passage fluidly connected to the primary pump, and at least one accumulator in selective fluid communication with at least one of the high- and low-pressure passages. A fan isolation valve is movable between a flow-passing position at which the fan motor is fluidly connected to the primary pump via the high- and low-pressure passages, and flow-blocking position at which the motor is substantially isolated from the primary pump. Efficiencies in an electro-hydraulic charging system are improved by allowing the fan motor to be isolated during energy recovery operations.
The present disclosure further improves upon the efficiency of electro-hydraulic charging systems in order to more fully utilize all resources in an electro-hydraulic system.